Vector modulation or I/Q modulation is a method of modulating a carrier with two baseband signals, referred to as the I (in-phase) and Q (quadrature-phase) components. Use of such modulation allows for an efficient and accurate means of transmitting information on a modulated carrier wave. A typical transmitter may have two baseband sources, one for the I channel and one for Q channel.
Ideally, the I channel and Q channel will have exactly the same gain across all frequencies of interest. Moreover, in quadrature modulation the Q channel should be exactly 90 degrees out of phase with the I channel across all frequencies of interest. However, due to problems such as component mismatches and design flaws, there may be amplitude and phase imbalance between the I and Q channels.
The imbalance can lead to signal contamination that can show up as degradation in the quality of a signal affecting such quality metrics as Error Vector Magnitude (EVM) or Bit Error Rate (BER). Hence, it is desirable to be able to accurately and quickly measure the imbalance. Typically, the measurements are made before the I/Q modulator is sold to a customer. A number of conventional techniques exist for measuring this imbalance. However, all are understood to suffer from one or more of the following deficiencies.
Some I/Q imbalance measurement techniques are not very accurate. For example, the accuracy of some conventional techniques hinges upon the ability to very accurately measure a large difference between an upper and lower sideband signal. Unfortunately, a very small difference in the measured value affects the result significantly. One prior art technique measures at the output of the modulator the amplitude and phase of a constructive signal over a destructive signal, which typically have a very large difference between them. The measuring equipment errors over such a large difference can be significant when expressed as a percentage of the imbalance to be measured.
For example, it may be that the constructive signal is measured to be 100 times the amplitude of the destructive signal. However, when calculating the imbalance of the I/Q modulator, even a small error in the relative magnitudes of the signals greatly affects the imbalance calculation. Hence, the technique is not very accurate.
Some solutions to determine I/Q imbalance are tedious, as they require numerous measurements to be made. For example, some conventional solutions use a single sine wave to measure only one frequency at a time of the signal output by the vector signal modulator that is the device under test (DUT). However, for a typical DUT the imbalance is a function of frequency. Thus, many measurements need to be made to characterize the imbalance over a range of frequencies.
Yet another problem with conventional techniques is a failure to stimulate the DUT with a signal that is representative of a signal used in actual operation of the DUT. Thus, these techniques fail to detect problems such as thermal and compression effects. For example, modern communications signals have considerably different statistics from a sine wave, which is conventionally used to stimulate a DUT. A realistic signal will subject the components of an I/Q modulator to subtle thermal effects. Moreover, the output signal of the I/Q modulator is subject to compression when there are brief peaks in the input signal. Testing the DUT with a conventional constant magnitude sine wave at a single frequency will not cause subtle thermal and compression effects. Failing to detect these effects results in an imbalance measurement that does not reflect the actual imbalance that the DUT will experience under actual use.
Thus, one problem with conventional methods for measuring I/Q imbalance is the lack of accuracy due to, for example, an inherently large error in measurement. Another problem with conventional methods for measuring I/Q imbalance is that too many time consuming and tedious measurements need to be taken. Another problem with conventional methods for measuring I/Q imbalance is that the DUT is not stimulated with realistic signals and hence the imbalance measurements do not accurately reflect DUT problems, such as thermal and compression effects.